[ARM] Move Triple::getARMCPUForArch into ARMTargetParser

This is very backend specific so either belongs in Toolchains/ARM or in
ARMTargetParser. Since it is used in lldb, ARMTargetParser made more sense.

This is part of an effort to move information about ARM/AArch64 architecture
versions, extensions and CPUs into their respective TargetParsers.

Differential Revision: https://reviews.llvm.org/D137564